A membrane electrode assembly (MEA) for a fuel cell includes an ionomer membrane which is coated with catalyst layers on opposite sides. Gas diffusion medium (GDM) layers are attached to or abut against the respective catalyst layers. In the fuel cell, a bipolar plate having flow field channels abuts against each GDM layer for the flow of reaction and by-product gases.
For multiple reasons, it is preferred to integrate the catalyst layers and the ionomer membrane of the MEA with the gas diffusion medium (GDM) layers into a single component which is known as a unitized electrode assembly (UEA). As the GDM layers are attached to the MEA, several criteria must be met. First, the attachment process must not alter the properties of the ionomer membrane or catalyst. This requires precise control of the bonding temperature and pressure.
Second, the attachment process must not alter the properties of the GDM layers. This requires avoidance of gross mistakes such as denting, scratching and tearing of the GDM, as well as precise control of attachment process parameters such as the magnitude of pressure applied to the GDM. Third, the interface between the GDM and each catalyst layer must remain unaffected by the attachment process.
Fourth, if adhesive is used to attach the GDM to the MEA, the adhesive must not adversely affect GDM performance. Fifth, the GDM must be precisely positioned with respect to the MEA during attachment. If the attachment process is capable of only marginal tolerances, the UEA will cause an overall fuel cell size increase. Additionally, there are performance and stability issues associated with large tolerances in GDM placement.
There are several important considerations which relate to the application of adhesive to the GDM. One of these considerations is the need to facilitate precise positioning of the adhesive on the GDM, since such positioning can affect the performance, size and durability of the fuel cell. Another important consideration involves the need to apply a layer of adhesive having minimal thickness such that compression of or local stress concentration in the active area of the MEA is avoided. Furthermore, the adhesive must be compatible with a fuel cell environment and must not introduce any contaminants to the MEA or GDM.
Therefore, a fabrication process for a UEA is needed which satisfies the above-mentioned criteria and results in a UEA that is characterized by enhanced tolerances and performance in a fuel cell.